The electrowinning of a metal from a compound thereof dissolved in an electrolyte is usually followed by a purification process of the product metal. In order to minimise the subsequent purification process, the metal is advantageously electrowon in an environment which contains no or little elements (or species thereof) that are liable to contaminate the produced metal. In commercial metal electrowinning, contamination of the product metal is minimised by avoiding the introduction of contaminating elements into the electrolyte, in particular by controlling the purity of the raw material that is used.
In the field of aluminium electrowinning the contamination of the product aluminium is due to the impurities present in the raw material, usually alumina containing a small amount of iron oxide, and to elements found in the structure of the aluminium electrowinning cell that dissolve during operation in the electrolyte, for example sulphur or nickel found in carbon anodes.
With the development of non-carbon aluminium electrowinning anodes and the operation of cells without crust and ledge, the likelihood of contaminating the product aluminium by elements from the cell structure has significantly increased.
It is known to produce aluminium with a low contamination level by purifying the product aluminium after electrowinning, for example by degassing the molten aluminium outside the aluminium electrowinning cell as disclosed in U.S. Pat. No. 4,668,351 (Dewing/Reesor), as well as in WO00/63630 (Holz/Duruz), WO01/42168 (de Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora), WO02/096830 (Duruz/Nguyen/de Nora) and WO02/096831 (Nguyen/de Nora).
There is a great incentive to use non-carbon anodes to improve the aluminium production process by reducing pollution and the cost of aluminium production. Many proposals have been made to replace carbon anodes which are still commonly used in industry by non-carbon anodes.
The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.
Only recently has it become possible to produce metal-based anodes that can resist the cell's environment for several hundred hours and even longer and that are sufficiently electrically conductive so as to permit commercial use. These recent developments, in particular anodes made of an electrically conductive metal anode core with an oxide-based active outer part, have been disclosed in several patents, such as, U.S. Pat. No. 6,077,415 (Duruz/de Nora), U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. Nos. 6,113,758, 6,248,227, 6,361,681 (all de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora), U.S. Pat. No. 6,379,526 (de Nora/Duruz), U.S. Pat. No. 6,521,115 (Duruz/de Nora/Crottaz), U.S. Pat. No. 6,562,224 (Crottaz/Duruz) and PCT applications, WO00/40783, WO01/42534 (both de Nora/Duruz), WO01/42536 (Duruz/Nguyen/de Nora), WO02/070786 (Nguyen/de Nora) and WO02/083990 (de Nora/Nguyen), WO02/083991 (Nguyen/de Nora), WO03/014420 (Nguyen/Duruz/de Nora), WO03/078695 (Nguyen/de Nora), WO03/087435 (Nguyen/de Nora), WO2004/018731 (Nguyen/de Nora), WO2004/024994 (Nguyen/de Nora), WO2004/044268 (Appourchaux/Nguyen/de Nora).
The replacement of carbon anodes by metal-based anodes leads to the presence of anode metal species dissolved in the electrolyte and reduced in the cathodic product aluminium. It has been proposed to prevent contamination of the product aluminium with an unacceptable amount of such metal species by operating the cell under strictly controlled conditions, as described in some of the above references, as well as in U.S. Pat. No. 6,540,887 (de Nora), U.S. Pat. No. 6,521,116 (Duruz/de Nora/Crottaz), U.S. Pat. No. 6,572,757 (de Nora/Berclaz), and PCT applications WO00/40781 (de Nora), WO01/31086 (de Nora/Duruz), WO01/42535 (Duruz/de Nora), WO02/097167 (Nguyen/de Nora), WO03/006716 (de Nora), WO03/006717 (Berclaz/Duruz), WO03/023092 (de Nora), and US publication 2003/0075454 (de Nora/Duruz).
US2004/0020786 (LaCamera et al.) published Feb. 5, 2004 discloses removal of sulphur from the electrolyte of an aluminium production cell in order to increase the cell's current efficiency. In several embodiments a purifying electrode is used in the electrolyte to remove the sulphur. Such an electrode is hidden behind a wall in an oxygen-free zone outside the main electrolyte stream to avoid exposure to anodically evolved oxygen. This publication recognises that iron impurities are disadvantageous for the current efficiency, particularly in combination with sulphur, but discloses only a method to remove sulphur and not iron.
As mentioned above, alumina that is used as the raw material for the commercial electrowinning of aluminium usually contains about 500-1000 ppm iron species which during electrowinning are reduced at the cathode and contaminate the product aluminium. It is not possible to limit iron contamination originating from the alumina feed by the methods described in the above mentioned references. The electrolyte of an aluminium electrowinning cell usually contains small quantities of contaminating impurities, typically up to 500 ppm iron and below 200 ppm nickel and possibly other elements, which should not be collected in the electrowon aluminium. There remains a need for reducing the contamination of aluminium during electrowinning.